This invention relates to reducing the time and cost of conditioning semiconductor process chambers operating at low pressures and more particularly to conditioning the surfaces or walls of a chamber with respect to adsorbed molecules, molecular fragments and/or atoms thereon.
As semiconductor microelectronics device feature sizes shrink to deep-sub micron dimensions, the new generation etch tools for semiconductor manufacturing will be operating in a high plasma density and low pressure regime. At low pressure, e.g. below 20 mTorr., the effects of gas interactions with the chamber side walls take on an increased importance in affecting the chemistry in the vicinity of a wafer being etched when compared to older generation tools that typically operate above 50 mTorr. and at a lower plasma density. These wall interactions include adsorption, collisions of energetic species, heterogeneous surface reactions, and desorption. With decreasing pressure, the concentration of the reactant species decreases while molecules, molecular fragments and/or atoms coming from the walls have greater access to the region of the wafer and the wafer itself due to the increased mean-free path between molecular and/or atomic collisions. Thus slowly desorbing molecular and atomic species from previous processing or new species resulting from chemical interactions on the chamber walls can have an increased influence on the wafer etching process.
To achieve a repeatable plasma process, the chamber wall conditions have to be kept to as close to an equilibrium state as possible. The conventional method for doing this is to periodically condition the plasma chamber in place of a production run, either after scheduled plasma clean procedures (between every few process runs up to a few days of process runs), after manual wet cleans, or after switching between processes using different chemistries.
The chamber cleaning process is designed to remove the chemical components from the previous process or to clean the residual film deposited on the inside walls of the chamber. For example, for a typical polysilicon or silicide gate etch process, this cleaning step involves running a plasma discharge with a mixture of Cl2, NF3, and O2. In addition to removing residual films from the walls of the chamber, this chemistry results in the displacement of chemical components, such as Br, that were adsorbed on the chamber walls. The removal of residual films and the displacement of chemical components leaves the chamber walls in a state that is far from the chemical equilibrium that is finally realized when the actual Clxe2x80x94 and/or Brxe2x80x94 containing etch process is subsequently run.
Leaving the walls in a state that is far from the chemical equilibrium is not desirable because it changes the conditions of the plasma and consequently the eching parameters. In order to assure a controlled process, a stabilization procedure (conditioning/seasoning) is performed. It consists of etching several dummy wafers (usually 20 or more) using process conditions very similar to those that will be used when running product wafers. The goal is to stabilize the partial pressures of the gas components during subsequent production by bringing the chamber walls into an equilibrium state of adsorption and desorption of the species relevant to the process. This seasoning process has been monitored using a mass spectrometer (residual gas analyzer or RGA) to measure the partial pressure of the relevant molecular species. Data from the mass spectrometer has verified that approaching equilibrium in the partial pressures of various species usually requires a conditioning/seasoning step of 20 or more wafers lasting for more than 30 minutes.
These clean/conditioning processes use a significant amount of nonproductive tool time in addition to the cost of the process chemicals and wafers. Consequently, reduction of nonproductive tool time and these costs provides a significant opportunity for increasing the productivity of the new-generation high-density plasma low-pressure etch tools being used in semiconductor fabrication.
In accordance with the present invention, a method for conditioning a process chamber that employs adsorbing gases at low pressures is described comprising the steps of introducing a first gas into the chamber, soaking the chamber in the first gas at a pressure in the range from 10 mTorr. to 5 Torr., continuing the step of soaking until desorption from the walls of the process chamber of residual chemicals species such as molecules, molecular fragments and/or atoms and the adsorption of first components of the first gas such as molecules, molecular fragments and/or atoms reaches a saturation concentration on the walls.